Heat transfer apparatus

ABSTRACT

1,088,869. Heating air by waste gases. BABCOCK &amp; WILCOX CO. Nov. 26, 1965 [Nov. 27, 1964], No. 50311/65. Heading F4S. In an air heater generally as described in Specification 988,353 in which a combustion air stream for a steam generator is heated as it flows serially over banks (61, 62, 63, Fig. 1) of flue tubes, the tubes extend between upper and lower tube plates and the upper tube plates 58A, 58B of the tube banks 61, 62 are carried in a common supporting structure which is cooled by air bled from the main combustion air stream. This cooling air reduces thermal stresses in the supporting structure caused by the temperature differential between the tubes of the respective banks. As shown, the supporting structure includes ducts 102, 102A for the cooling air. These ducts extend around the perimeter of each of the upper tube plates 58A, 58B. The tube plates are freely movable axially of the tubes and are connected to the structure by flexible bellows 90A, 90B which are protected from ash &amp;e. in the flue gases by shields 92A, 92B. Air tapped from the main stream passing over the tubes, flows through a narrow gap provided between each tube plate edge and adjacent walls 88A, 88B of the supporting structure, to the space behind each bellows member, whereby the latter is kept cool and may therefore be made of a relatively cheap steel. The cooling air passes thence to the ducts 102, 102A by way of apertures 98 in the walls 88A, 88B; and is discharged from the ducts through an outlet 110.

Dec. 27, 1966 R. P. SIEGFRIED ETAL 3,294,160

HEAT TRANSFER APPARATUS Filed Nov. 27, 1964 I 2 Sheets-Sheet 1 FIG.1

INVENTORS Rlchard P S|egf ned BY Frank E.Garr|son ATTORNEY United StatesPatent Ofiice Patented Dec. 27, 1966 3,294,160 HEAT TRANSFER APPARATUSRichard P. Siegfried, Bath Township, near Akron, and Frank E. Garrison,Clinton, Ohio, assignors to The Babcock & Wilcox Company, New York,N.Y., a corporation of New Jersey Filed Nov. 27, 1964, Ser. No. 414,2172 Claims. (Cl. 165-83) This invention relates generally to heat transferapparatus, and more particularly to tubular air heaters wherein air forsupporting combustion in a furnace is indirectly heated by the gaseousproducts of combustion from the furnace.

A tubular air heater has a passageway through which air flows intocontact with several banks of tubes within the passageway so as to beheated thereby. The tubes serve as conduits for the hot gases exhaustedfrom the steam generating unit. Conventional air heaters of this typedirect the hot gases serially first through the tubes of the tube bankadjacent the air outlet of the passageway and then the gases, at reducedtemperature, flow through the tubes of the bank adjacent the air inlet.In order to overcome the problem of cold-end corrosion associated withthe dew point-temperature relationship of the gaseous constituents ithas been proposed in the copending application of Arthur M. Frendberg,Serial No. 268,985, filed March 29, 1963, now US. Patent No. 3,194,214,granted July 13, 1965, to route a small portion of the comparatively hotgases as received from the generating unit gas pass to a relativelysmall tube bank located at the air inlet, immediately upstream of themain air heater, for the purpose of preheating the entering air to atemperature sufiicient to minimize formation of corrosive productswithin the tubes at the air inlet to the air heater.

In apparatus constructed according to the cited Fre-ndberg patent, thepreheater section is thus located adjacent the main air heater gasoutlet section which, during normal operation, will be handling gaseswhich are at a very much lower temperature than the gases in theneighboring preheater. This temperature difference between the adjacentsections may exceed 500 F. and it will be readily understood that thisdifferential would produce severe thermal Stresses in the structuralcomponents unless adequate provision is made to counteract their effect.It is therefore the principal object of the present invention tominimize such thermally induced stresses in the apparatus by minimizingthe temperature differential between the aforementioned air heatersections.

The problem as set forth is solved, according to the present invention,by providing a duct structure which extends about the periphery of theupper tube sheet of the preheater tube section with means for directingair from the air heater inlet duct into the aforementioned duct andacross the expansion joint associated with the preheater upper tubesheet. This air cools the expansion joint and the surrounding structure,and thereafter may be put to further use, such as for creating an airseal or as elutriating air of a shot cleaning system (not shown).

Various other objects, features and advantages of the invention willappear more fully from the detailed description which follows, taken inconnection with the accompanying drawings forming a part of the presentinvention, and in which:

FIG. 1 is a partially diagrammatic, sectional view of a vapor generatingand superheating unit having an air heater embodying the presentinvention;

FIG. 2 is a fragmentary plan view of the air heater of FIG. 1, on anenlarged scale;

FIG. 3 is a sectional side view of the air heater, taken along line 33of FIG. 2; and

FIG. 4 is an enlarged vertical sectional view of a portion of the airheater shown in FIG. 3.

Although it is not so limited, the invention, as illustrated in thedrawings, has been applied to an air heater used in conjunction with aforced-flow, once-through vapor generating and superheating unit.

Referring to FIG. 1, the illustrated unit includes wall structure 10defining a vertically elongated furnace chamber 12 having a gas outlet14 at the top of the furnace chamber opening to a horizontally extendinggas pass 16, the latter communicating at its rear end with the upper endof an upright gas passage 18. At the lower portion of the furnacechamber 12 is a fuel firing section consisting of horizontally extendingcyclone furnaces 20 which burn fuel and discharge high temperaturegaseous products of combustion into the lower portion of the furnacechamber 12. The gas pass 16 contains secondary superheater platensections 22 and reheater platen sections 24 arranged in series withrespect to gas flow. The gas passage 18 contains primary superheatersections 26 and economizer sections 28, also arranged in series withrespect to gas flow. A cavity 30 is provided between the economizersections 28 and the primary superheater sections 26 to facilitatewithdrawal of flue gases from the main gas stream. If desired, fluegases can be withdrawn from between the economizer sections 28. Thelower end of the gas passage 18 may incorporate an ash hopper 32 with agas outlet 34 which leads to the air heater 4%, and although not shown,the unit may have a suitable fan and ductwork between the gas passage 18and tie furnace chamber 12 to provide gas recirculation.

The air heater 46, as shown, includes a housing or casing 42, at thesides of the air heater, defining an air passageway 41 having anassociated inlet 44 connected with an inlet duct 46 to a forced draftfan 48. The air heater 40 is of the bottom-supported type, beingsupported on girders 43 and supporting columns 45, as shown in FIG. 1.The fan 48 supplies pressurized atmospheric air via inlet 44, which isheated in the passageway 41, then discharged through the outlet 50 andconveyed to the furnace windboxes 52 by hot air duct 54. The air flowingin the passageway 41 is indirectly heated by contact with spacedsuccessive parallel rows of upright gas conducting tubes 56A, 56B, and56C which are secured at opposite ends thereof, as by rolling, to upperhorizontal tube sheets 58A, 58B, and 58C and lower horizontal tubesheets 60A, 69B, and 60C, respectively. The tube rows in passageway 41are grouped within the housing 42 in three separate, successive tubebanks 61, 62, and 63, with the relatively small preheater tube bank 61comprising tubes 56A positioned adjacent the air inlet 44, the tube bank63 comprising tubes 56C positioned adjacent the air outlet 50, and thetube bank 62 comprising tubes 56B disposed between the tube banks 61 and63. There is one cavity 64 between the first and second tube banks 61and 62, and another cavity 66 between the second and third tube banks 62and 63.

The combustion products from the furnaces 20, after traveling firstupwardly through the furnace chamber 12 and then horizontally throughthe gas pass 16, next turn and proceed downwardly through the gaspassage 18. The main stream of flue gas leaves the gas passage 18through the gas outlet 34 and thereafter is turned downwardly withinduct 68 for passage through the tubes 56C of the third tube bank 63,after which it enters an ash hopper 70. The latter is defined bydownwardly and inwardly sloping bottom walls 72 of the air heaterhousing 42. The gas leaving the tubes 56C is deflected by a baffle 74toward the bottom of the ash hopper 70.

A smaller but hotter stream of flue gas is withdrawn from the mainstream of flue gas at the cavity 30, next conducted by bypass conduit 76to the first tube bank 61 for downward flow through the tubes 56A, andupon leaving the tubes is deflected by baffle 78 toward the bottom ofthe ash hopper 70. The gas from the tubes 56A combines with the mainstream of flue gas which has passed through the tubes 56C. The resultantcombined gas stream flows upwardly from the ash hopper 70, between thebafiies 74 and 78, into and through the tubes of the intermediate tubebank 62. It then is conducted by an exhaust duct 80 through a dustcollector 82 and subsequently discharged to the atmosphere.

The arrangement, as described, is intended to initially preheat therelatively cool atmospheric air supply entering the main air heater inorder to minimize the possibility of cold-end corrosion. The corrosionattack on air heater tubing results from the condensation of corrosivegases, such as sulfur compounds and water vapor, constitutents presentin the gases, as the relatively hotter gases come into contact with thetube metal which is being cooled by the colder air entering the airheater inlet. In addition, the moisture present on the interior surfaceof the tubing will also tend to collect corrosive dust or ash particlesentrained in the flue g-as flowing through the tubes, with the resultthat in addition to corrosion effects the accumulation of dust particlesrestricts the flow area within the tubes and decreases the heat transferefficiency of the tubing.

In order to facilitate the identification of similar parts associatedwith the several tube banks 61, 62, and 63, the letter A will follow thereference numeral designating each component and associated partrelating to the preheater tube bank 61; and, likewise, the letters B andC will be employed following correspondingly similar parts designated bythe same basic reference numeral of associated and component parts ofthe main air heater tube banks 62 and 63, respectively.

Air entering air heater passageway 41, on its way to the cyclonefurnaces 20, is at relatively low temperature, say 100 F. Thisrelatively cool air is preheated by the high temperature flue gas takenfrom the cavity 30 between the primary superheater sections 26 and theeconomizer sections 28, and it is conveyed through by-pass conduit 76 tothe tubes 56A adjacent the air heater inlet 44. This by-pass gas may beat a temperature in excess of 850 F. entering the tubes 56A to providetube metal temperatures well above the corrosion limit of 230 F. or so,while at the same time quickly raising the temperature of the air abovethe temperature range conducive to promoting corrosion. The next tubebank 62 in the flow path of the air will have gas flowing therethroughat a temperature in the range of between 480 F. and 300 F. so as tofurther elevate the temperature of the air. Finally, the temperature ofthe gas flowing through the tube bank 63 will be in the range of between500 F. and 680 F. for raising the temperature of the air still further.

It can be seen from the foregoing that with the temperatures of theupper tube sheets 58A and 58B tending to approach the temperature of thegases they are contacting, the tube sheet temperatures will be of theorder of 850 F. and 300 F. respectively, with a resultant temperaturedifferential of the order of 550 F. As will be explained hereinafter,supporting structure associated with these tube sheets may havecorresponding temperatures, and it is imperative to reduce thistemperature differential in order to prevent distortion of thesupporting frame structure.

Supporting structure, as shown in FIGS. 2, 3, and 4 of the drawings,comprises horizontal channel members, including channel members 86, 88A,and 88B. Longitudinal channel members 86 extend in parallel relationshiplengthwise of the air heater passageway 41 substantially for the lengthof the air heater 40. Transverse channel members 88A are disposed onopposite sides of the tube sheet 58A; and transverse channel members 883are disposed on opposite sides of the upper tube sheet 53B. Thesetransverse channel members 88A and 88B are connected at theirextremities to the respective webs of the longitudinal channel members86.

As shown in the drawings, the upper tube sheet 58A of the preheater tubebank 61 is bounded on all four sides by channel members, on two oppositesides by transverse channel members 88A, and on the two other oppositesides by extended portions of the channel members 86 so as to define aregion which provides an extension conduit 89 of the by-pass conduit 76.Heating gas flows downwardly through conduits 76 and 89 into the upper,open ends of the tubes 56A which are secured to and project through thetube sheet 58A.

Disposed within the extension conduit 89 is an expansion joint A whichserves to effect a gas-tight seal between the by-pass conduit 76 and thetube sheet 58A.

The expansion joint 90A is suitably shielded to minimize accumulationsof ash entrained in the flue gases by telescoping parts comprising ashort upstanding collar 91A disposed about the periphery of the tubesheet 58A, and a cooperating sleeve 92A closely spaced therefrom andconnected to the back face of the webs of the channel members 86 and88A. Thus the collar 91A and the sleeve 92A are in verticallyoverlapping relationship, with a sliding fit between them which permitsthe tube sheet 58A to move in a vertical direction as the tubes 56Aundergo expansion and contraction due to changes in the gas and airtemperatures, while at the same time shielding the expansion joint 90Afrom ash particles entrained in the flue gas stream. Furthermore, sleeve92A serves as an additional pressure barrier between the passageway 41and the conduit 76; and the collar 91A reinforces the sleeve 92A.

The upper extension of the peripheral expansion joint 90A is secured tothe back face of the web of channel members 86 and 88A, and the lowerextended portion thereof is secured to an angle iron frame 96A whichextends about the periphery of the tube sheet 58A. The expansion joint90A is suitably provided with spaced folds or leaves disposed betweenthe upper and lower extended portions to permit vertical expansion andcontraction of the joint while maintaining the aforementioned seal.Thus, the tube sheet 58A is freely movable in vertical direction whilethe expansion joint 90A effectively prevents the corningling of air andflue gas.

In order to cool the expansion joint 90A and the transverse channelmembers 88A and 88B, provision is made for purposely leaking relativelycool inlet air from the passageway 41 .past the edges of the upper tubesheet 58A into the region between the expansion joint 90A and the backsurfaces of the channel members 86 and 88A, after which the cooling airpasses through apertures 98 in the webs of these channel members, asshown.

In order to keep the tube sheet 58A in horizontally spaced relationshipwith the channel members 86 and 88A, vertically extending spacer barsare arranged about the edges of the upper tube sheet 58A and secured tothe back surfaces of the mentioned channel members. The relationship ofspacer bars 93 and the edge of the upper tube sheet 58A is shown in FIG.4.

By reducing the environmental temperature at the expansion joint 90A,this part can be made of low cost carbon steel material, rather than ofalloy steel material. Furthermore, the same cooling air which cools theexpansion joint also cools the transverse channel members 88A and 88B ofadjacent tube banks 61 and 62, thereby maintaining them at substantiallythe same temperature and preventing unequal thermal expansion in thesupporting structure of the air heater. Without this provision forcooling the neighboring transverse channel members, there would be atemperature difference upwards of 550 F. existing between the twoadjacent tube banks. It will therefore be appreciated that distortiondue to unequal thermal expansion is avoided by the present arrangement.

Referring now especially to FIG. 3, the specific transverse spacedchannel members 88A and 8813 which are adjacent tube banks 61 and 62,respectively, have their flanges extending toward one another. Upper andlower plates, 100 and 101, respectively, secured to the extending legsof the channels are employed to close the space between the flanges ofthese channel members 88A and 88B to thereby define a duct 102. Theseplates 1'00 and 101 are preferably secured to the flanges by welding inorder to make the duct 102 air-tight. The duct 102 is continued aboutthe entire periphery of the expansion joint 90A by means of a skirt 104of U-shaped cross section having its free edge portions joined, as bywelding, in abutting relationship with the flanges of the channelmembers 86 and 88A. It is noted that the extension of the duct 102provided by the skirt 104, is on the outer sides of the channel members86 and 88A; and, in order to provide communication with the firstmentioned portions of the duct 102, a rectangular opening 106 is formedin the web of the longitudinal channel member 86, as shown in FIGS. 3and 4.

With this arrangement, air can flow from the air passageway 41 throughthe regions between the spacer bars 93, then into contact with the outersurface of the expansion joint 90A, and through the apertures 98 intothe duct 102 for cooling the channel members 86, 88A, and 88B, afterwhich the air exists through an exhaust pipe 110.

Theair which is exhausted through pipe 110 may be beneficially used,such as for sealing air, or as elutriating air for a shot cleaningsystem, with the pressure differential existing between the pressure inair passageway 41 and that at the point of use producing the air flow.

From the foregoing, it can be seen that the air heater 40 includes apreheating tube bank 61 ahead of the main air heater tube banks 62 and63. Further, the air heater 40 has been provided with means for coolingthe expansion joint 90A conducting by-pass gas to the preheater tubebank 61, and which cools supporting members so as to prevent atemperature differential between adjacent supporting members. This isaccomplished by utilizing inlet air flowing through duct structureformed in part by the supporting members. It is an advantage of thepresent invention that the air heater casing can be secured directly tothe supporting structure without the necessity of expansion jointstherein. Furthermore, as mentioned previously, the expansion jointitself may be made from less expansive materials, even when by-pass fluegases of relatively high temperature are directed to the preheater tubebank 61.

Although the invention has been shown in but one form, it will beobvious to those skilled in the art that it is not so limited, but it issusceptible of various changes and modifications without departing fromthe spirit thereof as covered by the claims.

What is claimed is:

1. An air heater comprising:

an air passageway,

first, second and third tube banks each including:

a multiplicity of tubes and a tube sheet supporting said tubes, meansfor moving relatively low temperature air through said passageway intosuccessive contact with the outer surfaces of the tubes of said first,second and third tube banks,

means for directing relatively high temperature heating gas throughtubes of said first and second tube banks and through tubes of saidthird and second tube banks, whereby heat is transferred to the aircontacting said tubes, supporting means connected to the tube sheets ofat least two neighboring tube banks and extending along the adjacentedges thereof, means defining with said supporting means a duct open tosaid passageway for conducting at least a portion of the air flowingthrough said passageway into heat transfer relationship with saidsupporting means, and an expansion joint between said supporting meansand the tube sheet of said first tube bank providing a flexible sealbetween the air flowing through said duct and the heating gas flowingthrough said directing means. 2. An air heater comprising: ahorizontally elongated air passageway having an inlet and an outlet,first, second and third tube banks each including:

a multiplicty of upright tubes and upper and lower horizontal tubesheets supporting said tubes, said first tube bank being positionedadjacent to said inlet and said third tube bank being positionedadjacent to said outlet, means for moving relatively low temperature airthrough said passageway into successive contact with the outer surfacesof the tubes of said first, second and third tube banks, means fordirecting one stream of relatively high temperature heating gasdownwardly through tubes of said first tube bank and upwardly throughtubes of said second tube bank and for directing another stream ofrelatively high temperature heating gas downwardly through tubes of saidthird tube bank and upwardly through tubes of said second tube bank,supporting means including horizontal members connected to the tubesheet of said first tube bank and extending about the edges thereof,means defining with said members a duct open to said passageway forconducting at least a portion of the air from said inlet therethroughinto heat transfer relationship with said supporting means, and anexpansion joint flexibly connecting said members and edge portions ofthe tube sheet of said first tube bank and providing a seal between saidone stream of heating gas and the air flowing between said passagewayand said duct.

References Cited by the Examiner UNITED STATES PATENTS 1,780,294 11/1930Davis -82 2,828,946 4/1958 Smith 165-83 2,965,358 12/1960 Behrens et a1l65-82 FOREIGN PATENTS 1,211,918 10/ 1959 France.

734,008 7/1955 Great Britain.

ROBERT A. OLEARY, Primary Examiner. I, W. STREULE, JR., AssistantExaminer,

1. AN AIR HEATER COMPRISING: AN AIR PASSAGEWAY, FIRST, SECOND AND THIRDTUBE BANKS EACH INCLUDING: A MULTIPLICITY OF TUBES AND A TUBE SHEETSUPPORTING SAID TUBES, MEANS FOR MOVING RELATIVELY LOW TEMPERATURE AIRTHROUGH SAID PASSAGEWAY INTO SUCCESSIVE CONTACT WITH THE OUTER SURFACESOF THE TUBES OF SAID FIRST, SECOND AND THIRD TUBE BANKS, MEANS FORDIRECTING RELATIVELY HIGH TEMPERATURE HEATING GAS THROUGH TUBES OF SAIDFIRST AND SECOND TUBE BANKS AND THROUGH TUBES OF SAID THIRD AND SECONDTUBE BANKS, WHEREBY HEAT IS TRANSFERRED TO THE AIR CONTACTING SAIDTUBES, SUPPORTING MEANS CONNECTED TO THE TUBE SHEETS OF AT LEAST TWONEIGHBORING TUBE BANKS AND EXTENDING ALONG THE ADJACENT EDGES THEREOF,MEANS DEFINING WITH SAID SUPPORTING MEANS A DUCT OPEN TO SAID PASSAGEWAYFOR CONDUCTING AT LEAST A PORTION OF THE AIR FLOWING THROUGH SAIDPASSAGEWAY INTO